The invention generally relates to fluid inlets for ingesting fluid from a stream moving relative to a body in which the inlet is located, and more particularly to flow modifying structures for enhancing the flow into fluid inlets such as found on aircraft (air intakes) and on ships (water intakes).
It is known that the intake efficiency of a fluid inlet opening into the surface of a body deteriorates with increasing thickness of the viscous boundary layer on the surface ahead of the inlet. The decrease in efficiency is attributed to ingestion of slow moving, low energy boundary layer fluid into the inlet. A higher-energy, freely flowing stream of fluid exists further out from the surface of the body, beyond the boundary layer.
For an inlet that is flush with the surface, such as a flush-type air inlet on the fuselage of an aircraft, the boundary layer may result in the inlet receiving only a fraction of the total pressure (static plus dynamic pressure) existing in the free-stream, outside the boundary layer. The same considerations apply to flush-type water inlets on the hulls of ships.
Efforts have been made to improve the intake efficiency of flush inlets, and in the case of aircraft, some improvement is obtained by shaping the inlet to disturb the boundary layer flow in a manner that increases the flow into the inlet. One commonly used inlet configuration has a generally elongate shape, oriented parallel to the stream flow, and having sides that commence at a pointed forward end and diverge rearwardly to an enlarged aft portion. The rearwardly divergent sides of the inlet tend to set up counterrotating vortices as the air flows over the edges of the sides into the inlet duct, and these counterrotating vortices tend to draw higher-velocity air adjacent the boundary layer into the inlet.
While shaping of the inlet in the above manner does improve the inlet efficiency in some applications, the improvement is effective only where the airflow past the inlet has a relatively thin boundary layer. Such is not the case for air inlets that are located near the aft end of an aircraft fuselage. Because the boundary layer on the exterior surface of the fuselage increases progressively from the forward to the aft end, the vortex action of the specially shaped inlet is unable to penetrate the thick boundary layer that develops adjacent the aft portion of the fuselage.
Another, technique for improving inlet efficiency is to use a ram scoop mounted over the inlet to scoop air from the air stream into the inlet. Scoops do provide an improvement over unaided flush-type inlets, but they are accompanied by certain disadvantages that limit their effectiveness. The primary disadvantage is that the scoop must be mounted so as to retractable into a flush relation with the fuselage for eliminating or minimizing drag when the inlet is not being used. It is difficult and expensive to provide a retractable scoop, especially where the scoop must be retrofitted to an existing inlet. Also, a scoop has a tendency to merely capture the lower energy boundary layer air existing between the fuselage surface and the outermost extent of the scoop rather than entraining the higher energy air flowing above the boundary layer. Furthermore, the static air pressure in front of the scoop tends to increase above the static pressure that exists in the scoop's absence and this increased static pressure exacerbates the boundary layer build-up and further inhibits flow into the inlet.
Accordingly, an object of the invention is to overcome the above-mentioned shortcomings associated with flush-type inlets by improving their efficiency, particularly when a thick, viscous boundary layer exists adjacent the surface of the body in which the inlet is defined.
Another object is to improve the efficiency of flush-type inlets by modifying the flow adjacent the inlet and doing so without unduly increasing the drag between the stream flow and the body in which the inlet is located.
Still another object is to provide a flow modifying airfoil for improving the efficiency of flow into an existing, flush-type air inlet on the fuselage of an aircraft, wherein the airfoil is easily retrofitted to an existing inlet.